The present invention relates to novel herbicidal aqueous based microemulsions and methods for preparing the microemulsions.
Aqueous pesticidal formulations have attracted considerable interest in recent years because they possess certain advantages over non-aqueous formulations. One advantage aqueous formulations (e.g. suspension concentrates, concentrated emulsions and flowables) possess includes the reduction or elimination of organic solvents. This reduction or elimination of organic solvents results in reduced phytotoxicity (encouraged by some organic solvents); decreased costs when compared to organic-based formulations; safer handling; better compatibility with a greater variety of packaging materials; and, in some instances, enhanced biological activity.
Concentrated aqueous emulsions are an alternative to emulsifiable concentrates. Concentrated emulsions are advantageous because of their increased stability over emulsifiable concentrates and ease of use. However, it is known that the high surface area created in preparations of this sort is usually accompanied by large surface free energies. This, in turn, creates the opportunity for a variety of breakdown processes, and therefore, while the kinetic stability of these emulsions (dispersions) may be increased, they still represent thermodynamically-unstable systems.
Microemulsions present a unique class of thermodynamically-stable liquid dispersions. The thermodynamic stability of microemulsions is attributed to the presence of near zero interfacial tensions at equilibrium and also a minimum or potentially negative Gibbs free-energy term for the system. In order to achieve low interfacial tensions, the use of several surfactants is usually required. When one of the surfactants is soluble in the water phase and the other is soluble in the organic phase, each one has only a marginal effect on the other, and their combined effect may be large enough to reduce the interfacial tension to near zero at finite concentrations. As a rule microemulsions are difficult to formulate and are obtainable only with certain surfactant combinations and only within specific finite concentrations of these surfactant combinations. However, because microemulsions offer the greatest potential stability for formulations of this kind, there is an ongoing search in the art for microemulsion compositions.